This invention is an improvement in bridge measurement circuits which provides such circuits with a linear output when used with single-arm resistance-type transducers and also to provide linear output of nonlinear resistance-type transducers when desired.
FIG. 1 shows a conventional Wheatstone bridge circuit commonly used for measurement of resistance changes in a single arm, resistance-type transducer (RTD), such as a platinum resistance thermometer The bridge has an excitation voltage source V connected at node 1 to a first branch of the bridge which comprises two series connected resistances, resistance R.sub.A and transducer resistance R.sub.B (RTD) with resistance R.sub.B connected to ground at node 2. Voltage V is also connected at node 1 to a second branch of the bridge to two series connected resistances R.sub.C and R.sub.D With resistance R.sub.D being connected at node 2 to ground. The output signal voltage e.sub.o of the bridge is taken at node 8 (e.sub.a) between resistance R.sub.A and transducer resistance R.sub.B and between resistances R.sub.C and R.sub.D at node 4 (e.sub.b). The output signal voltage is amplified by output operational amplifier AR. A change in output signal voltage e.sub.o of the bridge circuit is a nonlinear function of the change in the resistance of transducer resistance R.sub.B. As shown in the calculations in Appendix 1, (1), only when the resistance of R.sub.A is much larger than the resistance of R.sub.B does the relationship approach linearity. However, the larger the resistance of R.sub.A becomes, the smaller the output signal voltage e.sub.o becomes. Therefore, a compromise must be made between linearity and output signal amplitude.
To overcome the problem of nonlinearity and small output signal, a modification to the bridge circuit can be made which is shown in FIG. 2.
In FIG. 2, node i is omitted and, for the first branch of the bridge, a current source i replaces resistance R.sub.A such that the current source i develops a voltage drop across transducer resistance R.sub.B. For the second branch of the bridge, a voltage source V is connected in series with resistance R.sub.C and resistance R.sub.D and resistance R.sub.D is connected to ground as in FIG. 1 except that resistance R.sub.C is variable. Again, output signal voltage e.sub.o is taken at node 8 (e.sub.a) between current source i and transducer resistance R.sub.B and at node 4 (e.sub.b) between resistances R.sub.C and R.sub.D. Output signal voltage e.sub.o is amplified by output operational amplifier AR. Variable resistance R.sub.C and resistance R.sub.D form a voltage divider for zero adjustment of the amplifier AR.
When the current source i is used to develop a voltage across the transducer resistance R.sub.B, and resistance R.sub.C is adjusted for output voltage signal e.sub.o to be equal to zero when the transducer resistance R.sub.B is at a zero setting, then a change in output signal e.sub.o versus the change in resistance in transducer resistance R.sub.B will be linear The disadvantage in the circuit of FIG. 2 is (1) a zero adjustment is required, and (2) the zero adjustment will not remain stable if either the current source i or the excitation voltage V were to change. Therefore, both the current source i and the voltage source V would have to be highly regulated to maintain the stability of the zero setting.
It is therefore an object of this invention to provide a bridge measurement circuit in which the zero setting does not depend on a current source or an excitation voltage and to provide a linear output from the bridge circuit when a single arm resistance-type transducer, such as a platinum temperature sensor, is used in the circuit.
Another object of this invention is to provide a simple way to linearize minor nonlinearities in the output of resistance-temperature transducers, such as that of a platinum temperature sensor.